fix some uses I missed

src/test/codegen/box-maybe-uninit.rs

@@ -12,5 +12,8 @@ pub fn box_uninitialized() -> Box<MaybeUninit<usize>> {
     // CHECK-NOT: alloca
     // CHECK-NOT: memcpy
     // CHECK-NOT: memset
-    Box::new(MaybeUninit::uninitialized())
+    Box::new(MaybeUninit::uninit())
 }
+
+// FIXME: add a test for a bigger box. Currently broken, see
+// https://github.com/rust-lang/rust/issues/58201.